The present invention relates to a nozzle of converging-diverging shape for creating mist flow at supersonic velocity, an inertia separator based thereon, and a method for the supersonic separation of a component of a predominantly gaseous stream. More in particular, the invention relates to the separation of one or more components from said stream by condensation of the selected components and subsequent separation thereof.
Separation can find applications in different industrial settings, such as in the oil and gas industry, in the chemical industry, in the paint industry, and a wide variety of other industries. Separation can be used in various industrial processes such as, for example, in removal of carbon dioxide from flue gas, in air-conditioning (water removal) and in the drying natural gas before its distribution into a network of pipelines.
Numerous methods and apparatus exist for separating components from a gaseous or other fluids. Examples of conventional separation apparatus include distillation columns, filters and membranes, settling tanks, centrifuges, electrostatic precipitators, dryers, chillers, cyclones, vortex tube separators and adsorbers. However, there are disadvantages and/or problems associated with each of these conventional apparatus which may make them undesirable for certain applications. In addition, various inertia separators have been described in the art, equipped with a supersonic nozzle.
JP-A-02,017,921 refers to the separation of a gaseous mixture through the use of supersonic flow. The device includes a swirler positioned upstream of a supersonic nozzle. The swirling fluid stream then passes through an axially symmetric expansion nozzle to form fine particles. The swirl is maintained over a lengthy axial distance, creating a large pressure drop. In order to separate a component from the three component gas flow, a large upstream swirl must be initially provided by the swirler and a significant amount of energy therefore must be input to the system.
U.S. Pat. No. 3,559,373 refers to a supersonic flow separator including a high pressure gas inlet, a rectangularly-shaped throat, and a U-shaped rectangular-cross sectional channel. The channel includes an outer curved permeable wall. A gas stream is provided to the gas inlet at subsonic speeds. The gas converges through the throat and expands into the channel, increasing the velocity to supersonic speed. The expansion of the flow in the supersonic region results in droplet coalescence and the larger droplets pass through the outer permeable wall and are collected in a chamber. The separation force, i.e., the force necessary to separate out the various phases of the flow stream, is dependent on the radius of the curvature of the channel. The radius of the curvature of the channel, however, is necessarily limited to prevent normal shock waves. Therefore, the shape of the device described in U.S. Pat. No. 3,559,373 inhibits the force available for separating out liquid droplets from the flow stream. Further, liquid droplets are not collected across the area of the channel.
EP-A-0,496,128 refers to a method and device for separating a gas from a gas mixture. The device includes a cylinder which converges to a nozzle and then diverges into a swirl zone. Gas enters an inlet port of the cylinder at subsonic speeds and flows through a converging section of the nozzle. The flow expands out of the converging section into the diverging section of the cylinder at supersonic velocity. A pair of deltoid plates impart a swirl to the supersonic flow. The combination of the supersonic velocities and the swirl assist in condensing and separating a condensed component from the gaseous components of the flow stream. An outlet pipe is positioned centrally within the cylinder to allow discharge of the gaseous components of the flow stream at supersonic velocity. The liquid components continue on through a second diverging section, which drops the velocity to subsonic, and through a fan, ultimately exiting the cylinder through a second outlet.
International application No. WO 99/01194 describes a similar method and corresponding device for removing a selected gaseous component from a stream of fluid containing a plurality of gaseous components. This device is equipped with a shock flow inducer downstream of the collecting zone so as to decrease the axial velocity of the stream to subsonic velocity. Application of a shock wave in this manner results in a more efficient separation of the formed particles.
These references describe various supersonic inertia separators, however, without a detailed description of the nozzle to be used.
Designs of nozzles suitable for inertia separators are shaped different from those that are used in jet engines, thrusters, etc. Both use convergent-divergent nozzles (Delaval Nozzle), which implies that in the meridian section a minimum diameter exists, referred to as the xe2x80x98nozzle throatxe2x80x99. However, the diverging section of nozzles used as a thrust device can be a simple conical diverging section (cf. Perry""s Chemical Engineers"" Handbook, 5-32). The shape of the diverging section to obtain supersonic mist-flow (i.e., a two phase comprising liquid/solid particles of condensed components of the stream present as fine particles transported with the gas phase) must be of a special shape; methods of design are given by Liepmann and Roshko (Elements of Gasdynamics, Wiley, New York, 1957, p. 284) the contents of which are incorporated by reference.
U.S. Pat. No. 5,261,242 concerns a process and apparatus for separation of solid particles or liquefied substance from their carrier fluid, using an inertial separator and, upstream of it, when needed, a nozzle system whose general function is to transform the fluid carrying the substance to extract into a rapid flow allowing to separate the substance as result of the inertial effect. According to this patent, a converging-diverging nozzle is to be applied having a particular profile (depicted in FIG. 2 of the US patent). The nozzle is said to be useful in the sector of energy recovery, industrial drying, drying of fluids carrying liquefiable substances and lowering of dew points of gases, of gas purification technology and aerosols separation and gas separation. Accordingly, this patent describes a nozzle of a convergent-divergent type having a channel comprising, in the direction of the fluid flow, upwards and downwards from a throat portion respectively a converging and a diverging channel portion, and wherein said nozzle channel has a profile in the vicinity of said throat portion upwardly and downwardly therefrom, shaped to cause the pressure and the flow rate in said zone to remain substantially constant over the nozzle channel axis.
However, it remains unclear what shape and dimensions this nozzle must have to achieve a separation efficiency of at least 15% (minimum separation efficiency for say air conditioning), preferably at least 50% (minimum separation efficiency for say natural gas treatment), and/or to provide separable particles of say 0.1 to 2.5 micrometer diameter.
SU-A-1768242 and SU-A-1722540 also disclose supersonic inertia separators without giving attention to the effect of the geometry of the nozzle on the particle growth and performance of the separator.
What is needed is a method and device which overcomes the disadvantages and insufficiencies of the prior separation methods to create and grow particles of a readily separable size with a limited amounts of external energy, rotating parts and pressure drop.
The present invention provides a nozzle of converging-diverging shape for creating mist flow at supersonic velocity comprising:
a throat having a characteristic diameter D*;
an inlet having a characteristic diameter D1, positioned a distance L1 upstream of the nozzle throat; and
an outlet having a characteristic diameter D2, positioned a distance L2 downstream of the nozzle throat, wherein the ratio of L2/(D2xe2x88x92D*) is larger than 50, but smaller than 220.
In the above definition and throughout this specification the throat is the part of the nozzle having the smallest flow area (dD/dx=0, wherein xe2x80x98dDxe2x80x99 is the incremental change in characteristic diameter and dx refers to the incremental change of position along the axial coordinate); the inlet is the subsonic inflow plane of the nozzle where it starts to converge (dD/dx£0), and the outlet is the supersonic outflow plane of the nozzle where it stops to diverge (dD/dx30). Likewise, the expression xe2x80x9ccharacteristicxe2x80x9d is used in the above definition and throughout this specification to define the diameter independent of the shape (circular, rectangular, etc.) of the cross-section i.e. the section normal to the nozzle axis. The characteristic diameter equals 4 times the cross-sectional area divided by the circumference.
The invention also provides an inertia separator for supersonic separation of a component of a predominantly gaseous stream, comprising a nozzle as described above, and a separation section downstream thereof having at least one outlet for the component that is to be separated, and at least one outlet for the remaining gaseous stream.
With the expression xe2x80x9cpredominantly gaseous streamxe2x80x9d in the above definition and throughout this specification is meant a stream that may have a minor amount of liquid or solid content, for example, a gas stream comprising 0-10%wt liquid and/or solid content.